Home Energy Nuclear Electricity Climate Change Lighting Control Contacts Links


XYLENE POWER LTD.

FNR INDICATOR TUBES

By Charles Rhodes, P.Eng., Ph.D.

INDICATOR TUBE PURPOSE:
The purpose of the indicator tubes is to indicate to the FNR monitoring and control systems: the vertical position of each movable fuel bundle, the gamma flux originating from each movable fuel bundle and the liquid sodium discharge temperature of each movable fuel bundle.
 

INDICATOR TUBE DESCRIPTION:
There are positively buoyant indicator tubes attached to the movable fuel bundles. These indicator tubes extend vertically 0.3 m to 1.4 m above the primary liquid sodium coolant surface. The indicator tubes are field attached to the movable fuel bundles.

Each indicator tube consists of a central thermally conducting tube open at the bottom with a flat top hat and a surrounding thermally isolating concentric sealed buoyant vacuum filled annular region. The annular region provides thermal isolation and ensures that the temperature of the liquid sodium in the central tube is approximately the same as the liquid sodium discharge temperature of the associated movable fuel bundle.

The outside tube, which is thermally isolated from the inside tube, must withstand the external primary liquid sodium head pressure. The top and bottom thermally insulating spacers between the inner tube and the outer tube must keep the annular region vacuum sealed.

The annular region, in combination with the fuel tube plenum regions, provides a path for gamma rays to pass through the primary liquid sodium bath. These gammma rays indicate the relative fission power of each movable fuel bundle.

The center of the indicator tube top hat has a small hole to vent gases trapped in the central tube.

The annular region of the indicator tube provides sufficient positive buoyancy so that when 1.4 m of the indicator tube is projecting above the primary liquid sodium surface the indicator tube maintains a firm upright position.

Note that the buoyancy of the annular region is not sufficient to lift the net weight of a movable fuel bundle when the indictor tube is fully immersed in liquid sodium.
 

THE TOP HAT:
The inner tube of each indicator tube assembly extends above the outer tube. At the top each inner tube has a 14 inch diameter flat thermally conductive top hat which is thermally attached to the inner tube. This top hat has a small central hole that allows gas trapped within the inner tube to escape.

The bottom of the top hat is thermally insulated. Its flat upper surface emits infrared radiation which indicates its temperature and hence the temperature of the movable fuel bundle liquid sodium This temperature measurement precision and reproducibility is an important aspect of power FNR safety.

In operation the liquid sodium level in the indicator tube central tube is approximately the same as the liquid sodium level outside the indicator tube.
 

LASER TARGET:
Each indicator tube presents a 14 inch diameter round elevation target to an overhead laser scanner.

Note that when the movable fuel bundles are fully inserted into the matrix of fixed fuel bundles the indicator tube top hats are about 1.4 m above the primary liquid sodium coolant surface.
 

TEMPERATURE INDICATORS:
This 14 inch diameter target also emits IR radiation indicating its temperature.
 

GAMMA RAYS:
Gamma radiation passes up inside the fuel tuble plenums, through the fuel tube top plugs, through the liquid sodium at the indicator tube attachment point, through the bottom annular insulator, up the annular space in the indicator tube, through the top annular insulator and then through the indicator tube top hat.
 

INDICATOR TUBE:
There are 7.1 m high buoyant indicator tubes field attached to the movable active fuel bundles. The vertical position of each active movable fuel bundle is visually indicated by the 0.3 m to 1.4 m exposed height of the top of its indicator tube above the primary liquid sodium surface.
 

INDICATOR TUBE ATTACHMENT:
Indicator tubes are attached to the movable fuel bundle lifting points after the movable fuel bundles are installed and are removed before the movable fuel bundles are relocated. The indicator tube attachment point is the movable fuel bundle lifting point. The indicator tubes must have dual J type bottom hooks for attachment to the lifting points of the movable fuel bundles.
 

LIFTING POINT:
A FNR fuel bundle lifting point is achieved by replacing the (3 / 16) inch thick diagonal plates with (3 / 8) inch thick diagonal plates in the upper portion of the fuel bundle where there are fuel tube plenums. The two (3 / 8) inch thick diagonal plates extend above the tops of the fuel tubes. Two 3.0 inch diameter holes in each plate form the lifting point.

The diagonal plates connecting each fuel bundle lifting point to the corresponding fuel bundle corner girders must also allow unobstructed primary liquid sodium flow and must not prevent individual fuel tube insertion or extraction.
 

INDICATOR TUBE MATERIAL AND DIMENSIONS:
The indicator tube diameter should be minimal to minimize obstruction of the natural liquid sodium circulation, but must be sufficient to allow accurate steady state movable fuel bundle liquid sodium discharge temperature measurement.

The indicator tubes are fabricated from HT-9 steel (85% Fe, 12% Cr, 1% Mo, 0% C, 0% Ni). The tube plus hook places the top of the indicator tube 7.5 m above the top 0f the movable fuel bundles. Hence the indicator tube itself is only about 7 m long.

The height allowances for the fixed fuel bundle components from bottom to top are: legs (1.5 m), bottom grating (0.1 m), fuel tubes (6 m), lifting point (0.3 m), swelling allowance 0.1 m. Hence the fuel bundle shipping container and the air lock tube must be able to accommodate a fuel bundle with an overall length of 8.0 m. This same air lock is long enough to accommosate the indicator tubes.
 

Indicator Tube: 4.0 inch OD X 0.____ inch wall X 7.5 m long_______
Mass = Pi X 4.0 inch X 0.____ inch X 7.5 m X (.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= ____ kg
 

VERTICAL THERMAL EXPANSION:
Note that the open steel lattice near the bottom of the primary liquid sodium pool and the fuel bundles will thermally expand vertically with increasing surrounding liquid sodium temperature. During normal reactor operation the open steel lattice is likely to be about 120 degrees C cooler than the liquid sodium temperature at the top of the fuel bundle. The thermal expansion will be significant and will affect the calculation of the movable fuel bundle insertion into the matrix of fixed fuel bundles unless temperature compensating measurements are performed. Hence the overhead laser scanner needs a compensating fixed fuel bundle elevation measurement.

The differential vertical thermal expansion per fuel bundle is approximately:
20 ppm / deg C X 430 deg C X 16.5 m = 0.1419 m
Hence it is essential that the laser scanning system cancel out vertical thermal expansion.  

MOVABLE FUEL BUNDLE TRAVEL LIMIT:
The movable fuel bundle vertical travel is limited to 1.1 m by the probe length (1.2 m) and the hydraulic cylinder end piece and piston thickness (0.3 m) and height of the steel lattice (1.5 m) and at the top by a hydraulic actuator vent hole.

The extent of insertion of a movable fuel bundle into the fixed fuel bundle matrix is determined by the volume of liquid sodium inside that fuel bundle's hydraulic actuator. There is fluid pressure feedback which indicates the approximate movable fuel bundle vertical position due to the changing buoyancy of the indicator tube. The hydraulic fluid feed tube is routed through the open steel lattice. This hydraulic tube must be sufficiently flexible to allow for +/- 1.0 m ______earthquake induced movement of the fuel assembly with respect to the primary pool walls.
 

This web page last updated February 6, 2022.

Home Energy Nuclear Electricity Climate Change Lighting Control Contacts Links